The preliminary work for the study of the effects of alcohol consumption upon phenytoin metabolism in rat liver has been completed. The conditions for the in vitro metabolism of phenytoin by the rat liver microsomal fraction and for quantification of the parahydroxylated metabolite of phenytoin have been established. Since phenytoin is a drug that exhibits undesirable side effects at concentrations just above those producing the desired anti-convulsant effect, understanding the mechanism by which phenytoin blood concentrations are affected by alcohol consumption should improve therapy with this drug.